Electrically fired explosive fasteners



Aug. 13, 1968 F. M. WILLIS ELECTRICAL-LY FIRED EXPLOSIVE FASTENERS F I 6 IA Filed Sept. 29, 1967 HTI INVENTOR FRANK ll. WILLIS ATTORNEY United States Patent 3,396,623 ELECTRICALLY FIRED EXPLOSIV E FASTENERS Frank Marsden Willis, Sewell, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Sept. 29, 1967, Ser. No. 671,773 5 Claims. (Cl. 85-65) ABSTRACT OF THE DISCLOSURE Electrically fired, explosively actuated fasteners such as rivets, which have an electrode extending substantially axially through the head portion of the fastener body, are provided with an annular, substantially coaxial groove in the face of the head portion to facilitate alignment of the fastener-firing device.

Background of the invention Electrically fired explosive fasteners such as rivets, bolts, screws, nails, pins, etc., were recently described in US. 3,332,311, issued July 25, 1967. These fasteners typically comprise an electrically conductive body having a head portion and shank portion; a substantially axial cavity in the shank portion; a peripherally insulated, nonventing electrode extending substantially axially through the head portion and terminating in the cavity; and a semiconductive explosive charge comprising a mixture of heat-sensitive detonating explosive and finely divided electrically conductive material in the cavity bridging the internal end of the electrode and the cavity Wall, thereby completing the integral fastener energizing circuit. Such fasteners, particularly rivets, are gaining widespread use in the fastening of skin to airplane bodies and in the assembly of refrigerators, vehicle bodies and similar heavyduty objects.

In operation, these fasteners are positioned in predrilled holes in the elements to be connected and then individually fired by a fastener-firing device which simultaneously grounds the fastener body and discharges to its central electrode sufiicient current to actuate the explosive charge. Actuation occurs within 50 microseconds and expands the shank of the fastener which locks the pieces to be joined firmly together but doesnt split or rupture the fastener body. Suitable fastener-firing devices are exemplified in Sosnowicz & Willis, US. application Ser. No. 521,641, filed Jan. 19, 1966, and now US. Patent No. 3,305,118, the teachings of which are included in this application by reference. Such firing devices comprise a nosepiece having a metallic, tubular grounding nose (positioning means) for engagement with the head of the fastener, a pin-like metal probe (electrode) slidably mounted within the grounding nose for movement along its axis and engagement with the electrode of the fastener, and an electric circuit including a power source and condenser connected to the electrode. The grounding nose normally is spring-biased into its forward-most position ahead of the probe so that the probe is not exposed. In practice, the operator of the firing device first engages the head of the fastener with the grounding nose and then forces the device toward the fastener causing the spring to compress and the probe to move forward relative to the grounding nose and into contact with the electrode.

In order for current to pass from the firing device to the fastener, the probe of the firing device must contact the electrode in the fasteners head without also contacting the head itself, in which event the fastener would be shortcircuited. Thus, rather precise alignment of the probe with the electrode is required. Obtaining such alignment is difiicult for two reasons. First, the diameter of the fasteners electrode is quite small, e.g., it may be as small as about 0.03 inch, and the end of the probe must be of even smaller diameter; hence, there is little margin for error in alignment. Second, it has been found that if the angle between the axis of the probe and the axis of the electrode (hereinafter the misalignment angle) is more than about 8, the grounding nose tends to slip off the head of the fastener, particularly when the operator exerts the pressure required to cause the above-noted forward movement of the probe. Consequently, there is no discharge of current from the firing device to the electrode of the fastener, and the explosive charge in the fastener is not actuated. In this case, the firing device must be aligned more closely or reapplied to the fastener to insure a more positive alignment of the grounding nose with the fastener head. This is unduly time consuming for production line assembly techniques. Therefore, there is needed a simple and positive means to position the nosepiece of the firing device on the fastener head to insure the discharge of current to the fasteners electrode with the least time and inconvenience.

Summary of the invention In accordance with this invention, electrically fired explosive fasteners of the type described in US. 3,332,311

' are improved by being provided with simple and effective means to insure quick and positive positioning of a fastener-firing device on the fastener head with increased tolerance for angular misalignment. This means comprises an annular groove at least 0.001 inch deep which is situated in the face of the head portion, is substantially coaxial with the electrode extending through the head portion, and has a diameter and width adapted to receive the annular tip of a firing devices grounding element. Such an annular groove not only provides the positive seating required to prevent the grounding nose from slipping off the head portion when the operator applies pressure on the firing device to extend the probe, but also provides a hinge-type pivot between the head portion and grounding nose which increases the maximum tolerable misalignment angle from about 8 up to about 20, thus eliminating the need for substantially coaxially aligning the probe and fastener electrode. The increased tolerability in the misalignment angle insures a simple, reliable and rapid positioning of the grounding nose of the firing device on the head of the explosive fastener so that current can be discharged from the probe to the explosively actuated fastener.

Brief description of the drawing In the accompanying drawings which illustrate specific embodiments of this invention and wherein like numerals designate like elements:

FIGURES 1A and 1B are plan and cross-sectional views, respectively, of the preferred explosive fastener of this invention.

FIGURE 2 is a cross-sectional view of another embodiment of this invention with the nosepiece of the firing device seated in the annular groove.

FIGURE 3 is a cross-sectional View illustrating how the invention insures firing of the fastener albeit there is a substantial angle of misalignment between the axis of the probe and the axis of the fasteners electrode.

Detailed description of the invention The invention is best described in connection with the accompanying drawing. FIGURES 1A and 1B show a countersunk head-type rivet which is provided with annular groove 1 in face 2 of head portion 3. Head portion 3 and integral shank portion 4 comprise the rivet body which is of metal. It is seen that annular groove 1 is substantially coaxial with central non-ejecting electrode 5 which extends axially through portion 3 and terminates in cavity 6 in shank portion 4. The external end of this electrode is substantially flush with face 2 to give the head a smooth appearance. Electrode is insulated along its entire length by a flexible coating 5a, e.g., of Formvar insulating resin about 1 mil thick. Both ends of the electrode are bared of insulation. Semiconductive explosive charge 7 bridges the internal bare end of the electrode and the cavity wall thereby completing the integral rivet energizing circuit. The end of the cavity is closed by a spacer element 8 of readily deformable material and spherical metal plug 9 which is locked in place by swaging as at 10. Although such cavity-closure means are optional, their use is preferred where substantially noiseless fastener operation is desired.

In FIGURE 2, shank 4 of a brazier head-type rivet extends through aligned holes in plates 12 and 13 which are to be joined. Nosepiece 14 of a firing device is posi tioned on head portion 3 with grounding nose 15 in am nular groove 1 thereby grounding the rivet. Probe 16 is retracted within insulating sleeve 17 which separates the probe from the grounding nose. Sleeve 17 is fastened to grounding nose 16 by means not shown. What remains to be done to fire the rivet is for the firing devices operator to apply vertically downward pressure on the device thereby compressing the spring (not shown) that normally maintains the grounding nose and sleeve 17 in the position shown relative to the tip of the probe. Compression of the spring simultaneously causes the probe to move downward into contact with electrode 5. Upon such contact, current from a suitable source is discharged through probe 16 to electrode 5 thereby actuating explosive charge 7 which expands the rivet locking plates 12 and 13 together.

FIGURE 3 shows how annular groove 1 permits firing a countersunk head-type rivet albeit misalignment angle on is large enough that grounding nose 15 normally would slide across face 2 of head 3 when the operator applies downward pressure on the firing device to force probe 16 into the illustrated firing (current discharging) position. It can also be seen from this figure that annular groove 1 provides a hinge-type pivot for annular grounding nose 15, hence nosepiece 14. To illustrate, should misalignment angle on be large enough that extension of probe 16 causes it to hit to the side of electrode 5, thus preventing current discharge, the operator merely need pivot the firing device around the point of contact between groove 1 and grounding nose 15 until nosepiece 14 is sufi'iciently upright that contact between the probe and electrode can be made. A particular advantage of this hinge-type pivot arrangement is that in practice it will almost invariably cause automatic alignment of the probe and electrode. In other words, once any part of the tip of the grounding nose is engaged in annular groove 1, the downward pressure exerted by the operator automatically tends to pivot the firing device, hence the nosepiece, toward the vertical.

The advantages provided by the annular groove are best realized with fasteners whose heads have substantially fiat faces, i.e., either fiat faces or curved faces having a large radius of curvature.

The diameter and width of the groove are such that it will engage the annular tip of the grounding element. Thus, consideration must be given to the minimum diameter for a nosepiece 14 wherein grounding nose 15 and probe 16 are of sufficient thickness, hence strength, and wherein insulation 17 between the probe and grounding nose is of sufficient thickness, hence strength and resistance, to perform their intended functions. Taking all these factors into account, it has been discovered that the annular groove generally should have an outer diameter (O.D.) of at least 0.1 inch and a width of at least 0.016 inch. The inner diameter (I.D.) of the groove will, of course, be at least as large as the CD. of the insulation around electrode 5, and preferably is at least about 0.08 inch. Given these groove dimensions, a nosepiece of appropriate complementary configuration can readily be constructed.

The size of the groove preferably is such that it is equally suitable for all size fastener heads so that the same firing device can be used in all cases. The fasteners of interest generally have heads whose faces have a minimum dimension of at least about 0.22 inch (e.g., this the minimum diameter for round faces); therefore, the CD. of the groove preferably is less than about 0.20

inch.

The annular groove must be at least 0.001 inch deep to provide satisfactory engagement with the grounding nose, with depths of at least 0.002 inch being preferred. The maximum depth of the groove is not critical provided of course that it is not sufiicient to impair the requisite structural integrity of the fastener head. Generally, there is no advantage in having the depth of the groove exceed about 0.005 inch.

The particular shape of the grooves cross section is not critical. For example, the cross section may be rectangular, V-shaped, or arcuate. In any event, it should have about the same mean diameter as the annular tip of the grounding nose. To eliminate stress raisers, the internal surface or bottom of the groove preferably is curved as shown on the drawing. A radius of curvature of 0.015 to 0.025 inch has been found satisfactory for this purpose.

The particularly preferred embodiment of annular groove has an ID. of at least about 0.13 inch, an OD. less than about 0.18 inch, a width of about from 0.02 to 0.03 inch, a depth of about 0.003 inch, and a curved internal surface whose radius of curvature is within the above range. Such a groove is particularly adapted for use with a grounding nose whose tip has substantially the same configuration, thus giving it the dimensions believed to be near optimum for a wide range of fastener head designs.

The annular groove can be stamped or machined in the preformed fastener head, but preferably is made during the head-forming operation by providing the head-forming die with a raised annular rib of the desired configuration.

It will be understood that many different sizes and configurations of fasteners may be provided in accordance with the teachings of this invention. The body of these fasteners may be of any of the common metals which have the properties of strength, and malleability adapted to make them serviceable for such use. Brass, bronze, aluminum nickel and iron alloys are examples of suitable metals.

The following example illustrates a complete preferred embodiment of the invention.

Two hundred and fifty rivets are prepared using aluminum rivet blanks similar to that shown in FIGURE 1. In each rivet, the shank is .134 inch in outer diameter and .273 inch long. The countersunk head is .225 inch in outer diameter and 0.039 inch deep at the section connected to the shank. The annular groove in the face of the head is 0.003 inch in depth, 0.021 inch in width, 0.177 inch in outer diameter, and 0.135 inch in inner diameter. In addition, the bottom of the groove has a 0.020 inch radius of curvature. The cavity extending the length of the shank is stepped, being 0.040 inch in minor diameter at a section adjacent the head and 0.060 inch in major diameter. An aperture 0.033 inch in diameter extends vertically through the head of the blank, the aperture being coaxial with the longitudinal axis of the rivet blank, i.e., centered in the truncated cone-shaped head. An electrode of aluminum, 0.115 inch long and 0.031 inch in OD, including a l-mil thick coating of Formvar polyvinyl formal resin, is inserted into the aperture and set in place to give an electrode assembly as shown in FIGURES l to 3. The cavity is loaded from theopen end with 16 milligrams of a mixture consisting by weight of 65% dextrinated lead azide, 16% micronized graphite,

4% nitrostarch, 13% tetracene, and 2% oil furnace black (Cabots Vulcan XC72R). The mixture is then pressed at about 35,000 psi. to set the charge in place. The end of the shank opposite that in which the conductor extends is closed by a rubber spacer element and a spherical steel plug which is positioned in contact with the spacer. The walls of the rivet blank in the section about the plug and spacer are subsequently swaged to reduce the diameter of the rivet shank around the spacer and spherical plug.

The rivets are positioned in predrilled countersunk holes in an 80-mil-thick plate and individually fired by a hand gun having a grounding nose Whose tip is of the same configuration and mean diameter as the groove in the face of the rivets. The gun simultaneously grounds the rivet body and discharges to the central electrode the current stored by a 20 microfarad condenser charged by a 90 volt (DC) current. All rivets are fired on the first attempt, i.e., Without realigning the grounding nose, although misalignment angles of up to about 20 are experienced.

I claim:

1. In an electrically fired eXplOsive fastener comprising an electrically conductive body having a head portion and shank portion, an explosive charge within the shank portion, and a peripherally insulated electrode extending substantially axially through the head portion into contact with the explosive charge, the improvement comprising an annular groove at least 0.001 inch deep in the face of the head portion, said groove being substantially coaxial with said electrode and having a diameter and width adapted to receive the annular tip of the grounding nose of a fastener-firing device.

2. A fastener of claim 1 wherein the annular groove has an inner diameter of at least about 0.08 inch, an outer diameter of at least about 0.1 inch, a width of at least 0.016 inch, and a depth of at least 0.002 inch.

3. A fastener of claim 2 wherein the face of the head portion is substantially fiat and has a minimum dimension of at least 0.22 inch, and the annular groove has an outside diameter of less than about 0.20 inch.

4. The fastener of claim 1 wherein the face of the head portion is round, fiat, and has a diameter of at least about 0.22 inch, and the annular groove has an inner diameter of at least about 0.13 inch, an outer diameter less than about 0.18 inch, a width of about from 0.02 to 0.03 inch, a curved bottom having a radius curvature of 0.015 to 0.025 inch, and a depth of about 0.003 inch.

5. A fastener of claim 4 having a countersunk head.

References Cited UNITED STATES PATENTS 1,277,370 9/1918 Bovard.

1,286,381 12/1918 Mead 28720.3 2,080,220 5/1937 Butter et al. 37 2,492,590 12/ 1949 Nofzinger 85-65 2,693,972 11/ 1954 Lavarack 28720.3 2,842,022 7/ 1958 Semmion 8565 3,332,311 7/1967 Schulz 85--65 MARION PARSONS, JR., Primary Examiner.

1'7. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,396,623 August 13, 1968 Frank Marsden Willis It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, lines 48 to 49, cancel "and now U. 8. Patent No. 3,305,118,".

Signed and sealed this 10th day of February 1970.

(SEAL) Attest:

idward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR. ttesting Officer Commissioner of Patents 

